Noise caused by jet engine exhaust is generally considered to be proportional to at least the sixth power of the velocity of the exhaust. In a conventional turbofan engine the velocity of the hot centrally located primary flow exiting from the turbine is much higher than that of the surrounding cooler flow from the fan. In some engines, the flows are initially separated by a wall, or splitter, but are allowed to mix in a tailpipe section prior to passing through the nozzle exit plane. In certain turbofan engine installations, such as the JT8D Engine-727 airplane installation, the tailpipe length is not sufficient to allow satisfactory mixing. The higher velocity of the primary gases exiting the nozzle creates very high levels of jet noise in mixing with the ambient air.
Devices located in the region of the nozzle exit plane for mixing the jet engine exhaust with ambient air are known in the art. See for example, the U.S. Pat. Nos. to POULAS (2,934,889), to YOUNG (3,161,257), to YOUNG et al (3,153,319), to LABUSSIERE (3,613,827), and to DUVVURI (3,664,455). These systems are primarily concerned with creating flow patterns to enhance mixing between the exhaust stream tube and the ambient air in a region extending at least several nozzle diameters downstream of the nozzle exit plane. The U.S. Pat. No. to BODINE (2,944,623) describes a system to accomplish this same objective by imparting a spin to the exhaust gases aft of the choked throat of the nozzle and just forward of the nozzle exit plane. The U.S. Pat. No. to MACDONALD (3,647,020) describes radially aligned vanes located either well forward of the exit plane (FIGS. 1-7) or at the exit plane (FIGS. 8-12) to cause the exhaust gases to "whirl" to increase the "divergence" of the jet stream and therefore the rate of mixing with the ambient air. These devices all have a common primary purpose; viz., the creation of flow patterns to force mixing of the total engine exhaust stream tube with the ambient air in the region aft of the nozzle exit plane.
In contradistinction to the devices described above, the system of this invention is concerned with internal mixing of hot primary exhaust and cool fan airflows within the confines of a turbofan engine so that the exhaust stream tube has a relatively uniform velocity, substantially lower than the unmixed peak velocity, at the nozzle exit plane. The desirability of mixing these two airflows has been recognized by other investigators. See for example, the U.S. Pat. No. to RAW (3,557,830) in which guide vanes in the nature of scoops, or chutes, are employed to direct hot primary gases outwardly into the surrounding fan stream to mix the two fluid flows. One problem in such devices is a relatively high drag created by the low pressure region on the downstream face of the chute. The patent asserts that if the chutes are properly configured, a pair of small local vortices may occur over the downstream face of the chutes to relieve this problem by inducing flow of air into that region; this apparently in the manner of boundary layer control devices in common use on aircraft wing surfaces. The vortices thereby developed merely reduce the chute element drag loss but do not significantly cause mixing. This is allegedly done by the flow through the chute. Water tunnel flow visualization tests of such chute devices have shown that the extent of mixing of the two flow patterns leaves much to be desired.